1. Technical Field
The invention relates to the motor speed control of electric vehicles powered by multiple batteries, specifically the use of multiple relays (electromechanical or solid state) as a way of discretely switching additional batteries into the circuit when more motor speed is requested, to increase supplied voltage and thereby motor speed, under the control of a computer algorithm which ensures that the battery bank remains balanced.
2. Description of the Background Art
Contemporary electric vehicles use motor speed controllers built from a plurality of one or more power transistors, typically MOSFETs or IGBT(s). A plurality of batteries are connected together, typically in series, to achieve the higher voltages needed for the motor, and the most positive and most negative terminals of the battery bank are connected to the motor speed controller.
In the current state of the art for a DC motor controller, a sensor such as a potentiometer or optical encoder attached to the vehicle's accelerator pedal causes a circuit to modify the pulse width of a square wave which is amplified to control the switching of the MOSFETs or IGBT(s), thereby chopping at high frequency the entire current from the battery bank before it reaches the motor, and thereby controlling the mechanical speed of the motor. See, for example, Cudlitz, U.S. Pat. No. 4,471,276.
Power transistors that are capable of switching the high currents used in electric vehicle applications are expensive. At higher voltage or current ratings, arrays of power transistors must be used, thus increasing controller complexity and expense. Unless over-rated, power transistors can be fragile, and over voltage conditions such as a battery surface charge or ringing due to wiring inductance can exceed the power ratings of the transistors and cause cascading and, in some circuit topologies, catastrophic explosive failure of the controller. These power electronic components are also sensitive to heat, such that excessive load can cause failure due to self-generated heat.
Furthermore, the use of power transistors to switch the entire current flow from the main battery bank is not efficient. The transistors have inherent switching and/or conduction losses that cause power loss in the form of heat dissipation. This is inefficient, wasting energy that could be used for propulsion, and reducing both battery life and the maximum allowable time between charges, while also mandating a larger minimum size, weight, and cost for the battery bank.
Even an inefficiency of a few percent in the power transistors, as in the current state of the art, causes a significant production of waste heat when power levels of many hundred kilowatts are used, such as in high power electric vehicle applications. Because the devices are intolerant of high heat, excessive load can cause failure due to self-produced thermal energy. This is exacerbated in solid state components that have a forward voltage drop which grows with increasing temperature and, which are thus prone to thermal runaway. This necessitates cooling, typically by fans or water cooling, adding both complexity and expense to the controller system.
Historically, electric vehicles that were in service extensively in the early 20th century before the domination of the automobile marketplace by the internal combustion engine used different, and simpler, techniques for motor speed control.
In one approach, a very large resistive element or parallel plurality of resistive elements was used to dissipate energy from the battery pack before it reached the electric motor. The energy dissipation was in the form of heat, and this style of controller was highly inefficient because, whenever the motor was not operating at full throttle, a significant proportion of the energy being delivered by the battery pack was deliberately wasted. This approach had the undesirable side effect of greatly reducing vehicle range. In this approach, again the entire battery pack was connected to the motor through the resistive motor controller.
In another approach, individual batteries were switched into the circuit at different times, typically by a large multipole drum switch that incorporated resistive elements, (see, for example, Storer, U.S. Pat. No. 1,291,233) or, later, with arrays of contactor (see, for example, Moody, U.S. Pat. No. 3,984,744) or rectifier contactor (as used in a controller manufactured by Seecom) circuits. Again, this approach significantly compromises vehicle range because, during a typical drive requiring varying speeds, the batteries that were switched into the circuit earlier at lower throttle settings, are discharged sooner than those batteries switched in only rarely when full power is requested. Additionally, battery life is compromised by this approach because some batteries in the pack would consistently be used more than others. Unequally discharged batteries also complicate effective battery charging, especially if the unequally charged batteries are in a series string, as is typical.
In another historical approach, the battery bank was commutated, i.e. the battery bank was divided into two or more equal-sized groups which could be reconnected in series or in parallel under the control switch(es). This approach typically provided a very limited number of speed settings, and is thereby disadvantageous for practical reasons and considerations of safety.
While the problem of precise motor speed control has been addressed (see, for example, De Villeneuve, U.S. Pat. No. 4,309,645 and Mendenhall, U.S. Pat. No. 4,415,844), what is truly needed is a robust and low-cost controller for high power applications, such as electric vehicles, where a number of discrete throttle positions equal to the number of batteries in the system is an acceptable granularity.